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      MySQL学习笔记_day2
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    <h3 id="5-2-条件查询"><a href="#5-2-条件查询" class="headerlink" title="5.2 条件查询"></a>5.2 条件查询</h3><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token comment">#进阶2：条件查询</span>
<span class="token comment">/* 
语法：
	select 
		查询列表
	from
		表名
	where
		筛选条件
分类：
	一、按条件表达式筛选
	
	条件运算符：> &lt; = != &lt;> >= &lt;=
	
	二、按逻辑表达式筛选
	
	逻辑运算符：
		&amp;&amp; || ！
		and or not
	三、模糊查询
		like
		between and
		in
		is null|is not null

*/</span>
<span class="token comment">#一、按条件表达式筛选</span>
<span class="token comment">#案例1：查询工资>12000的员工信息</span>
<span class="token keyword">SELECT</span>
	<span class="token operator">*</span>
<span class="token keyword">FROM</span>
	employees
<span class="token keyword">WHERE</span>
	salary<span class="token operator">></span><span class="token number">12000</span><span class="token punctuation">;</span>

<span class="token comment">#案列2：查询部门编号不等于90号的员工名和部门编号</span>
<span class="token keyword">SELECT</span>
	last_name<span class="token punctuation">,</span>
	department_id<span class="token punctuation">,</span>
<span class="token keyword">FROM</span>
	employees
<span class="token keyword">WHERE</span>
	department_id<span class="token operator">&lt;></span><span class="token number">90</span><span class="token punctuation">;</span>

<span class="token comment">#二、按逻辑表达式筛选</span>
<span class="token comment">#案例1：查询工资在10000到20000之间的员工名、工资以及奖金</span>
<span class="token keyword">SELECT</span>
	last_name<span class="token punctuation">,</span>salary<span class="token punctuation">,</span>commission_pct
<span class="token keyword">FROM</span>
	employees
<span class="token keyword">WHERE</span>
	salary<span class="token operator">>=</span><span class="token number">10000</span> <span class="token operator">AND</span> salary<span class="token operator">&lt;=</span><span class="token number">20000</span><span class="token punctuation">;</span>
<span class="token comment">#案例2：查询部门编号不是在90到110之间，或者工资高于15000的员工信息</span>
<span class="token keyword">SELECT</span>
	<span class="token operator">*</span>
<span class="token keyword">FROM</span>
	employees
<span class="token keyword">WHERE</span>
	<span class="token operator">NOT</span><span class="token punctuation">(</span>department_id<span class="token operator">&lt;</span><span class="token number">90</span> <span class="token operator">AND</span> department_id<span class="token operator">></span><span class="token number">110</span><span class="token punctuation">)</span> <span class="token operator">OR</span> salary<span class="token operator">></span><span class="token number">15000</span><span class="token punctuation">;</span>

<span class="token comment">#三、模糊查询</span>
<span class="token comment">/* 
like
between and
in
is null|is not null

*/</span>
<span class="token comment">#1.like</span>
<span class="token comment">/*
特点：
①一般和通配符搭配使用
	通配符：
	% 任意多个字符，包含0个字符
	_ 任意单个字符
*/</span>
<span class="token comment">#案例1：查询员工中包含字符a的员工信息</span>
<span class="token keyword">SELECT</span>
	<span class="token operator">*</span>
<span class="token keyword">FROM</span>
	employees
<span class="token keyword">WHERE</span>
	last_name <span class="token operator">LIKE</span> <span class="token string">'%a%'</span><span class="token punctuation">;</span>  <span class="token comment">#abc</span>
<span class="token comment">#案例2：查询员工名中第三个字符为n，第五个字符为l的员工名和工资</span>
<span class="token keyword">SELECT</span>
	last_name<span class="token punctuation">,</span>salary
<span class="token keyword">FROM</span>
	employees
<span class="token keyword">WHERE</span>
	last_name <span class="token operator">LIKE</span> <span class="token string">'__n_l%'</span><span class="token punctuation">;</span>
<span class="token comment">#案例3：查询姓名中包含第二个字符为_的员工信息</span>
<span class="token keyword">SELECT</span> 
	<span class="token operator">*</span>
<span class="token keyword">FROM</span> 
	employees
<span class="token keyword">WHERE</span> 
	last_name <span class="token operator">LIKE</span> <span class="token string">'_\_%'</span><span class="token punctuation">;</span> <span class="token comment">#转义\</span>
<span class="token comment">#--------------------------------------------</span>
<span class="token keyword">SELECT</span> 
	<span class="token operator">*</span>
<span class="token keyword">FROM</span> 
	employees
<span class="token keyword">WHERE</span> 
	last_name <span class="token operator">LIKE</span> <span class="token string">'_$_%'</span> <span class="token keyword">ESCAPE</span> <span class="token string">'$'</span><span class="token punctuation">;</span> <span class="token comment">#自定义转义escape</span>

<span class="token comment">#2.between and/not between and</span>
<span class="token comment">/*
①使用between and 可以提高语句的
②包含临界值，不要调换顺序
*/</span>
<span class="token comment">#案例：查询员工编号在100到120之间的员工信息</span>
<span class="token keyword">SELECT</span> 
	<span class="token operator">*</span>
<span class="token keyword">FROM</span>
	employees
<span class="token keyword">WHERE</span> 
	employees_id <span class="token operator">BETWEEN</span> <span class="token number">100</span> <span class="token operator">AND</span> <span class="token number">120</span><span class="token punctuation">;</span>

<span class="token comment">#3.in</span>
<span class="token comment">/*
功能：查询某字段的值是否属于指定的列表之内
特点：
	①使用in提高语句简洁度
	②in列表的值类型必须统一

*/</span>

<span class="token comment">#案例1：查询部门编号是30/50/90的员工名、部门编号</span>

<span class="token keyword">SELECT</span> 
	last_name<span class="token punctuation">,</span>department_id
<span class="token keyword">FROM</span> 
	employees
<span class="token keyword">WHERE</span> 
	department_id <span class="token operator">IN</span><span class="token punctuation">(</span><span class="token number">30</span><span class="token punctuation">,</span><span class="token number">50</span><span class="token punctuation">,</span><span class="token number">90</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">#-----------------------------------</span>
<span class="token keyword">SELECT</span> 
	last_name<span class="token punctuation">,</span>department_id
<span class="token keyword">FROM</span> 
	employees
<span class="token keyword">WHERE</span> 
	department_id<span class="token operator">=</span><span class="token number">30</span> <span class="token operator">OR</span> department_id<span class="token operator">=</span><span class="token number">50</span> <span class="token operator">OR</span> department_id<span class="token operator">=</span><span class="token number">90</span><span class="token punctuation">;</span>
	
<span class="token comment">#案例2：查询工种编号不是SH_CLERK或IT_PROG的员工信息</span>
<span class="token keyword">SELECT</span> 
	<span class="token operator">*</span>
<span class="token keyword">FROM</span> 
	employees
<span class="token keyword">WHERE</span> 
	job_id <span class="token operator">NOT</span> <span class="token operator">IN</span><span class="token punctuation">(</span><span class="token string">'SH_CLERK'</span><span class="token punctuation">,</span><span class="token string">'IT_PROG'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">#--------------------------------------</span>
<span class="token keyword">SELECT</span> 
	<span class="token operator">*</span>
<span class="token keyword">FROM</span> 
	employees
<span class="token keyword">WHERE</span> 
	<span class="token operator">NOT</span><span class="token punctuation">(</span>job_id <span class="token operator">=</span><span class="token string">'SH_CLERK'</span> <span class="token operator">OR</span> job_id <span class="token operator">=</span> <span class="token string">'IT_PROG'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>


<span class="token comment">#4、is null/is not null</span>

<span class="token comment">#案例1：查询没有奖金的员工信息</span>
<span class="token keyword">SELECT</span> 
	<span class="token operator">*</span>
<span class="token keyword">FROM</span> 
	employees
<span class="token keyword">WHERE</span> 
	commission_pct <span class="token operator">IS</span> <span class="token boolean">NULL</span><span class="token punctuation">;</span>

<span class="token comment">#案例2：查询有奖金的员工信息</span>
<span class="token keyword">SELECT</span> 
	<span class="token operator">*</span>
<span class="token keyword">FROM</span> 
	employees
<span class="token keyword">WHERE</span> 
	commission_pct <span class="token operator">IS</span> <span class="token operator">NOT</span> <span class="token boolean">NULL</span><span class="token punctuation">;</span>

<span class="token comment">#----------------对比------------------------------------</span>
<span class="token operator">=</span>		只能判断普通的内容
<span class="token operator">IS</span>  <span class="token boolean">NULL</span>        只能判断<span class="token boolean">NULL</span>值
<span class="token operator">&lt;=></span>             安全等于，既能判断普通内容，又能判断<span class="token boolean">NULL</span>值<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h3 id="5-3-排序查询"><a href="#5-3-排序查询" class="headerlink" title="5. 3 排序查询"></a>5. 3 排序查询</h3><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token comment">#进阶3：排序查询</span>
<span class="token comment">/*
语法：
	select 查询列表
	from 表名
	【where 筛选条件】
	order by 排序列表【asc|desc】

执行顺序：
	①from 子句
	②where 子句
	③select 子句
	④order by 子句

特点：
1、asc代表的升序，desc代表的是降序，如果不写默认升序
2、order by子句中可以支持单个字段、多个字段、表达式、函数、别名
3、order by子句一般是放在查询语句的最后面，limit子句除外
*/</span>
<span class="token comment">#案例1：查询员工信息，要求工资冲高到底排序</span>
<span class="token keyword">SELECT</span> <span class="token operator">*</span> <span class="token keyword">FROM</span> employees <span class="token keyword">ORDER</span> <span class="token keyword">BY</span> salary <span class="token keyword">DESC</span><span class="token punctuation">;</span>
<span class="token keyword">SELECT</span> <span class="token operator">*</span> <span class="token keyword">FROM</span> employees <span class="token keyword">ORDER</span> <span class="token keyword">BY</span> salary<span class="token punctuation">;</span>

<span class="token comment">#案例2：查询部门编号>=90的员工信息，按入职时间的先后进行排序【添加筛选条件】</span>
<span class="token keyword">SELECT</span> <span class="token operator">*</span> 
<span class="token keyword">FROM</span> employees 
<span class="token keyword">WHERE</span> department_id<span class="token operator">>=</span><span class="token number">90</span>
<span class="token keyword">ORDER</span> <span class="token keyword">BY</span> hiredate <span class="token keyword">ASC</span><span class="token punctuation">;</span>

<span class="token comment">#案例3：按年薪的高低显示员工的信息和年薪【按表达式排序】</span>
<span class="token keyword">SELECT</span> <span class="token operator">*</span><span class="token punctuation">,</span>salary<span class="token operator">*</span><span class="token number">12</span><span class="token operator">*</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token operator">+</span>IFNULL<span class="token punctuation">(</span>commission_pct<span class="token punctuation">,</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">)</span> 年薪
<span class="token keyword">FROM</span> employees
<span class="token keyword">ORDER</span> <span class="token keyword">BY</span> salary<span class="token operator">*</span><span class="token number">12</span><span class="token operator">*</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token operator">+</span>IFNULL<span class="token punctuation">(</span>commission_pct<span class="token punctuation">,</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token keyword">DESC</span><span class="token punctuation">;</span>

<span class="token comment">#案例4：按年薪的高低显示员工的信息和年薪【按别名排序】</span>
<span class="token keyword">SELECT</span> <span class="token operator">*</span><span class="token punctuation">,</span>salary<span class="token operator">*</span><span class="token number">12</span><span class="token operator">*</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token operator">+</span>IFNULL<span class="token punctuation">(</span>commission_pct<span class="token punctuation">,</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">)</span> 年薪
<span class="token keyword">FROM</span> employees
<span class="token keyword">ORDER</span> <span class="token keyword">BY</span> 年薪 <span class="token keyword">DESC</span><span class="token punctuation">;</span>

<span class="token comment">#案例5：按姓名的长度显示员工的姓名和工资【按函数】</span>
<span class="token keyword">SELECT</span> LENGTH<span class="token punctuation">(</span>last_name<span class="token punctuation">)</span> 字节长度<span class="token punctuation">,</span>last_name<span class="token punctuation">,</span>salary
<span class="token keyword">FROM</span> employees
<span class="token keyword">ORDER</span> <span class="token keyword">BY</span> LENGTH<span class="token punctuation">(</span>last_name<span class="token punctuation">)</span> <span class="token keyword">DESC</span><span class="token punctuation">;</span>

<span class="token comment">#案例6：查询员工信息，要求先按工资升序，再按员工编号降序【按多个字段排序】</span>
<span class="token keyword">SELECT</span> <span class="token operator">*</span> <span class="token keyword">FROM</span> employees <span class="token keyword">ORDER</span> <span class="token keyword">BY</span> salary <span class="token keyword">ASC</span><span class="token punctuation">,</span>employees_id <span class="token keyword">DESC</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
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    <p><span style="font-weight: bold;">作者：</span><a target="_blank" rel="nofollow noopener noreferrer" href="https://hxf233333.gitee.io/about">hxf233333</a></p>
    <p><span style="font-weight: bold;">文章链接：</span><a target="_blank" rel="nofollow noopener noreferrer" href="https://hxf233333.gitee.io/2021/02/25/MySQL%E5%AD%A6%E4%B9%A0%E7%AC%94%E8%AE%B0_day2/">https://hxf233333.gitee.io/2021/02/25/MySQL%E5%AD%A6%E4%B9%A0%E7%AC%94%E8%AE%B0_day2/</a></p>
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